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Chapter
Lubricant and Lubricant Additives
Debashis Puhan
Abstract
Lubricants have been used by humans for thousands of years in their simple 
machines such as wheel-axle bearings and sledges. Modern machines are much 
more complicated and are composed of many different machine elements which are 
in relative motion under varying loads, speeds and temperatures. Industrial lubri-
cants are significant for all kinds of industries whether machine building, chemical, 
textile, wood, food-processing, automotive, or wind power. Today’s lubricants have 
evolved to a complex mixture of chemical structures that ensure not only lower 
friction but also provide various other functionality such as lower wear, improved 
heat transfer, sealing, as well as control of soot, impurities, sludge and deposit 
formation in the mechanical equipment. Lubricant research and development has 
become indispensable in automotive engines and drive trains as these have been 
rapidly advancing towards smaller sizes, increased power, better fuel economy and 
lesser emissions. Development of lubricant additives and lubricant formulation has 
led to extended service intervals, enhanced fuel efficiency and improved machine 
durability. Future trends of lubricant development and use in the Industry 4.0 era 
and rise of electric vehicles look promising where several stakeholders already have 
taken their first steps.
Keywords: lubricant, lubrication, lubricant additives, base oil, greases, 
solid lubricant
1. Introduction
Lubricants have been in use for hundreds of centuries and are essential to our 
survival. Natural lubricants such as saliva and synovial fluid lubricate the food 
for easy mastication and reduce wear and tear of our joints respectively. Cooking 
oils prevent sticking of food onto frying pans and baking trays at the same time as 
conducting heat. Ancient Egyptians used lubricants to slide large stone blocks for 
building the great pyramids while the Romans used lubricant on the axles of their 
chariots [1]. Ancient lubricants were plant and animal based natural oils. With 
the onset of industrial revolution and our reliance on metal-based machinery and 
engines, petroleum-based lubricants witnessed a growth.
Modern lubricants are far more complex and perform various other functions in 
addition to lubricating such as cleaning, cooling, and sealing. The primary func-
tion of most lubricants is to reduce friction and this property is known as lubricity. 
A lubricant can be used in solid form, semi-solid, liquid form or gaseous form. 
Examples of solid lubricants are graphite and Molybdenum disulphide (MoS2), 
semi-solid lubricants are greases, and liquid are automobile engine oil. Depending 
on the requirements of a said application, the physical state of lubricant is chosen. 
For example, in space environments where liquid lubrication is not feasible due to 
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vacuum, solid lubricants are chosen. Air bearing are preferred in applications in 
machine tool applications where precision is of primary importance such as cut-
ting and finishing of optical lenses. Greases are used where a liquid oil would not 
remain in position due to its tendency to flow or when a sealing action is needed to 
prevent water-ingress in addition to lubrication. Today’s lubricants are designed 
and packaged to meet specific requirements for specific applications by lubricant 
formulators. The lubricant for automobile transmission and drive train has different 
requirements to satisfy compared to lubricant for an internal combustion engine or 
turbines. Further depending upon the type of turbines viz. gas, steam or hydraulic, 
the lubricant needs to be designed.
2. Lubricant composition
Typically, industrial lubricants contain 70-90% base oils and the rest is addi-
tives [2]. Base oils impart primary vital properties of the lubricant such as viscosity, 
viscosity stability, thermal stability, solvency, low temperature flow and volatility, 
oxidation stability. Additives have been used in lubricating oil since the 1920s and 
the demand for lubrication has resulted in continuous growth in the size of the 
market (USD 14.35 billion in 2015) with huge investments in research and develop-
ment to design and formulate superior lubricants that meets present and future 
environmental regulations and consumer expectations. Despite this, lubricant 
formulation has mostly remained an art. This is because blending a new formula-
tion for optimizing viscosity and obtaining optimum performance through perfor-
mance tests for friction and emissions is much easier than testing for parameters, 
such as impact on engine wear, sludge build-up and piston cleanliness which require 
long duration engine tests. For example, during the development of Castrol’s engine 
oil, ‘Edge with Titanium Fluid Strength Technology’, over 2400 unique formulations 
were engine-tested for an equivalent of 1.9 million miles.
To understand the need for additives, one must understand the implication of the 
Stribeck curve shown in Figure 1. Machine elements such as engine bearings work 
in hydrodynamic lubrication regime where the major function of the lubricant is to 
maintain its viscosity at all temperatures while ensuring a thick fluid film to keep the 
two contacting surfaces in relative motion separated at all loads and speeds. Rolling 
element bearings work on elasto-hydrodynamic lubrication (EHL) where the con-
tacting surfaces deform elastically and there is a very thin film separation. Cams and 
tappets work in the boundary lubrication regime where there is substantial metal to 
metal contact. And in the reciprocating motion of engine piston rings, all four kinds 
of lubrication regime occur.
Classical lubrication theory assumes that a lubricating oil is a Newtonian fluid with 
a fixed viscosity and the contacting surfaces to be rigid. George Osborne Reynolds 
approached the fluid film hydrodynamic lubrication using mathematical and physical 
approach (1886) to predict friction, film thickness and load carrying capacity [3]. In 
the real world, oils undergo shear thinning and behave as a non-Newtonian fluid due 
to heat and pressure developed at the contact. Furthermore, real surfaces are rough 
and can undergo elastic as well as local plastic deformation under fluid pressure. The 
EHL theory was developed by H.M. Martin (in 1916) and later by Ertel (in 1939) and 
Grubin (in 1949), Petrusevich (in 1951), Dowson and Higginson (in 1959), Dowson 
and Hamrock (in 1977) for predicting traction, load carrying capacity and film thick-
ness in heavily loaded contacts [4]. The non-Newtonian behavior of thin films under 
high pressure and lower rolling speeds has guided lubricant formulators to consider 
the shear-stress/shear-strain behavior, pressure-viscosity dependence of the lubricant 
as these are closely linked to the molecular properties of the oil composition. On the 
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Stribeck curve, when the speed is very low, the friction is governed by the chemistry of 
the lubricant molecules i.e. the molecular structure and orientation. It was W. B. Hardy 
(in 1920), who coined the term boundary lubrication and later with Ida Doubleday (in 
1922) established the basic concepts of boundary lubrication theory [5]. The bound-
ary lubrication properties such asfriction and film thickness at the interface of two 
surfaces are affected by the force fields of molecules in relation to their structure and 
polarity. Hence, formulators add boundary film additives to reduce friction in this 
regime [6]. Additives must also reduce wear, as in this regime, the two surfaces in 
relative motion are in contact hence prone to significant wear. Furthermore, addi-
tives need to counter the side effects of continuous as well as intermittent use of the 
lubricant such as they need to control heat, deposit formation, prevent foam forma-
tion, prevent fouling and corrosion due to water ingress, prevent wear, increase film 
strength under concentrated contacts, control greenhouse gas emissions. Therefore, 
as high as 30% content of modern automotive lubricants are chemical additives 
while some industrial oils may only contain 1% or less.
2.1 Base oil
A typical petroleum-based oil with no additive is called a base oil or base stock 
[7]. The generation of base stock starts with the identification and selection of a 
good petroleum crude followed by atmospheric distillation, vacuum distillation and 
solvent processing. Solvent processing has two processes viz. solvent extraction, and 
solvent dewaxing where undesirable molecules are separated where as in hydropro-
cessing undesirable molecules are converted to desirable ones. Hydroprocessing a 
general term for conversion of less desirable crude fractions into good quality feed-
stock using catalyst and hydrogen at high temperature and pressure. This can be cat-
egorized as hydrofinishing, hydrotreating and hydrocracking according to increasing 
order of severity. Another process called hydrodewaxing is required to remove long 
chain linear paraffins, isomerize straight chains to branched chains which improves 
the pour point. Only 10 % of crudes are converted to base stocks for lubricants, 
hence the refinery owners call the shots regarding the choice of crudes to be con-
verted to base oils while balancing the cost, yield and demand relative to all the other 
refinery products. However, to satisfy lubricant performance demands under severe 
operating conditions, high quality base stocks are needed. The lubricant manufac-
tures (refinery owner can also be lube manufacturer) buy these base stocks and 
other chemical compounds and formulate their lubricant for example SAE15W40 or 
ILSAC GF-5, meeting standards set by Original Equipment Manufacturers (OEMs), 
Figure 1. 
Stribeck curve.
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professional bodies, and international institutions like American Petroleum Institute 
(API), International Lubricant Standardization and Approval Committee (ILSAC), 
Society of Automotive Engineers (SAE) to name a few.
Owing their origin to petroleum crudes, lubricant base stocks are also mix-
tures of long chain hydrocarbons containing three types of chemical groups. i.e. 
paraffins, naphthenes and aromatics. The paraffins can be further classified into 
branched or straight chains. The chain length and branching affects the melting 
point and crystallization temperature of the paraffins. During the production 
of lube base stock, most of the unsaturated bonds, paraffin wax and sulfur con-
tent are removed, however depending on the severity of hydroprocessing, some 
wax, unsaturates and sulfur may remain. Base Stocks have been classified into 5 
categories by the API according to the presence of saturates, sulfur content and 
viscosity as shown in Table 1. Group I, II and III are derived from petroleum crude 
while Group IV is reserved for Polyalphaolefins (PAO) which are synthesized from 
gaseous hydrocarbons. Group V is for all other base stocks that are not included in 
other four groups such as mineral based napthenics, synthetic esters, polyglycols, 
silicones, polybutenes, phosphate esters etc. These oils are designed for severe 
performance requirements.
2.1.1 Application based on properties of base oil
Group I base oils are processed by solvent processing in which wax and 
multiring aromatics are removed while some sulfur and aromatics remain. These 
base oils are typically used in marine and diesel engine oils, heat transfer oils, 
hydraulic oils, conventional greases, industrial gear oils, and machine tool oils. 
These oils have high solvency, very high viscosity but poor viscosity index. The 
maximum operating temperature is 93°C.
Group II base stocks have undergone catalytic conversion to remove wax, 
aromatics and sulfur compounds. These base oils are used as automotive engine oil, 
automatic transmission fluid, gear oils and turbine oils. The maximum operating 
temperature of these base stocks is 121°C.
Group III base oils are similar to Group II oils but have a higher viscosity index. 
The severity of hydroprocessing removes any ring structures, as a result, its solvency 
is poor. These base oils are used in premium passenger vehicles, automatic transmis-
sion fluids, and food grade lubricants. The maximum operating temperature is 121°C.
Group IV oils are synthetically produced Polyalphaolefins from low molecular 
weight organic material. They have uniform molecular structures. They provide 
excellent high temperature performance and oxidation stability and therefore 
used in high performance engine and gear applications, heavy duty industrial 
Type Saturates (%) Sulfur (%) Viscosity 
index
Group I Solvent refinedof any machinery. Lubricant additives are chemical components that need 
to blend well with the base oil to function as a single fluid. Additive manufactures 
often sell several additives combined into an additive package and diluted with 
a base oil at a higher concentration. The additive package is then dosed into the 
lubricant blend by the lubricant manufacturer at an appropriate treat rate to give 
the desired performance. The concentration of various additives is constrained by 
various factors such as their primary function (for example dispersancy, wear pro-
tection and so on), their synergetic or antagonistic behaviour with other additives, 
and regulations set by industry bodies.
In a nutshell, lubricant additives can be categorized into various kinds based on 
their general roles of performance improvement and service life extension. First 
category are additives that impart new properties to the lubricant also known as 
surface protective additives. Examples include antiwear additives, extreme pres-
sure additives, corrosion inhibitors, detergents and dispersants. The second kind 
of additives enhance the existing properties already present in the lubricant hence 
known as performance additives. Viscosity index improvers, viscosity modifiers, 
friction modifiers, pour point depressants belong to this type. The third type of 
additives known as lubricant protective additives, are the ones that counteract the 
negative effects or changes that take place during the service life of the lubricant. 
These include antifoamants and, antioxidants. In this chapter only the major types 
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of lubricant additives are discussed. Other additives such as demulsifiers, emulsi-
fier, biocides may be added depending on the intended applications.
2.2.1 Types of lubricant additives
2.2.1.1 Pour point depressants
As the name indicates, these are additives that reduce the pour point of the 
lubricant, i.e. the lubricant remains in liquid state and maintains its fluidity (pour-
ability) at lower temperatures than without these additives. Usually as temperature 
decreases, paraffin molecules in the oil start to crystallize as wax (below 50°C) and 
the oil loses its ability to flow by gravity or to be pumped under pressure. This also 
affects the viscosity of the oil. Additives such as alkylaromatic polymers and poly-
methacrylates prevent wax crystal growth by modifying the interface between the 
wax and the oil molecules, to a certain extent thus lowering the pour point by about 
20–30°F (11–17°C). These are present up to a fraction of a percent in all paraffin-
based lubricants that lubricate machine elements such as bearings, gears exposed 
to cold start and cold (winter) operating temperatures. Modern multi-grade engine 
oils/motor oils composed of partly synthetic oil and partly mineral oil along with 
these additives, have pourpoints as low as −32°C.
2.2.1.2 Viscosity index improvers
Viscosity index improvers (VII) also known as viscosity modifiers are addi-
tives that prevent the oil from losing its viscosity at high temperatures which is 
a natural tendency of any liquid. These additives are available in all shapes and 
sizes and quality [8]. Polymethylmethacrylates, olefin copolymers, hydrogenated 
poly(styrene-co-butadiene or isoprene), esterified polystyrene-co-maleic anhydride 
are commonly used VIIs. The large oil soluble flexible polymer molecules uncoil 
and spread out as temperature increases thereby increasing the viscosity as shown 
in Figure 2. Furthermore, their numerous branches entangle with those of other 
neighboring molecules. By doing this, these macromolecular structures can trap and 
control smaller oil molecules, thus increasing the viscosity of the lubricant.
Permanent and temporary shear thinning of VII-thickened formulations can 
also occur depending upon the quality of the VII. In heavy duty application, due 
to the large compressive pressure between the two mating surfaces, VII polymer 
molecules, tend to align with each other and get “squashed” or even get chopped 
to small pieces under high shear conditions. When the polymer coils elongate and 
become aligned in the direction of the flow the viscosity temporarily drops result-
ing in reduced oil film thickness. After the lubricant leaves the contact between 
Figure 2. 
Mechanism of VII.
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the mating parts, the polymer coils return to their original shape and the viscosity 
of the lubricant returns to normal. This phenomenon is referred to as temporary 
shear-thinning. However, under further high shear rates, the long and flexible 
polymer chains can be cut or ruptured or pulled and ripped apart into smaller chains 
by molecular scission. Unfortunately, once this has occurred, the broken polymer 
chains cannot re-form into the single large chain and this causes the oil to perma-
nently lose viscosity leading to a reduction in oil film thickness, oil film failure and 
an increase in wear. This phenomenon is referred to as permanent shear-thinning.
2.2.1.3 Anti-wear agent
Anti-wear additives are additives that prevent two-body wear of the metallic 
countersurfaces in the boundary lubrication regime where the film thickness is 
small and there is asperity - asperity contact. These additives are polar in nature 
which enables them to attach to the metallic surfaces followed by tribochemical 
or mechanochemical reactions to form an anti-wear film. This newly formed film 
undergoes wear and formation at the top layers thus protecting the underlying 
metallic surface. As these additives form films by chemical reactions, they get used 
up and the amount of antiwear additives present in the lubricant reduces with time. 
These are typically phosphorous compounds. Zinc dialkyldithiophosphate (ZDDP) 
is the most common, the most researched and has been used since the 1940s [9]. 
Its use has been reduced in passenger vehicles in the last decade due to zinc metal 
causing poisoning of the catalyst in the exhaust gas catalytic convertor. ZDDP also 
provide antioxidant and corrosion-inhibition properties to the lubricant. Owing 
to the multi functionality of ZDDP, finding its replacement has been challenging 
because molybdenum-based additive such MoDTC (molybdenum dithiocarbamate) 
or MoDDP (molybdenum dithiophosphate) molecules cannot work as antioxidant. 
On the other hand, ash-less antiwear additives such as hindered phenols and amines 
are very expensive and are required in larger quantities. Till date, ZDDP is consid-
ered as the most cost-effective antioxidant and antiwear additive available, and the 
alternatives are currently very expensive.
2.2.1.4 Antioxidants
Antioxidants or oxidation inhibitors prevent the oxidation of the components of 
the base oil there by increasing the life of the lubricant. Oxidation of the lubricant 
molecules occur at all temperatures but at higher temperatures, it is accelerated. The 
presence of wear particles, water, and other contaminants also promote Oxidation 
of the lubricant molecules which then leads to formation of acids and sludge. The 
acids may further cause corrosion in the metallic parts while the sludge formation 
increases the viscosity of the lubricant. Almost every lubricating oil and grease 
contains antioxidants and examples include Zincdialkyldithiophosphates, hindered 
phenols, sulphurized phenols, and aromatic amines. These compounds decompose 
peroxides and terminate free-radical reactions that occur in the lubricant. These are 
sacrificial in nature hence their quantity gets reduced with time.
2.2.1.5 Defoamants
Defoamants or antifoaming agents are additives that prevent the lubricant from 
forming a foam and speed up the collapse of the foam if it does form. Foaming occurs 
because of constant mixing of the oil with air or other gases leading to air entrapment. 
Foam disrupts cooling of parts as it is not a good conductor of heat. It reduces the load 
carryingcapacity and the lubricant flow leading to excessive engine wear. Silicone 
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polymers such as polymethylsiloxane at a few parts per million and organic copo-
lymers such as alkoxy aliphatic acids, polyalkoxyamines, polyethylene glycols, and 
branched polyvinyl ethers, at higher concentration are widely used in mineral oils. 
The antifoaming agents are essentially insoluble in the lubricant hence they need to be 
finely dispersed in the lubricant. These droplets attach themselves to the entrapped air 
bubbles and aid in forming bigger bubbles (via coalesce). The larger bubbles rise read-
ily to the surface followed by bursting to release the trapped air. Bursting occurs by 
thinning of the air bubble film as the additive spreads due to its low surface tension.
2.2.1.6 Friction modifiers
Friction modifiers are used in engine oil and transmission oil to alter the coef-
ficient of friction that would be experienced between the sliding parts when only 
the base oil is present. Friction reduction results in improved fuel economy. Organic 
and sulfurised fatty acids, amines, amides, imides, high molecular weight organic 
phosphorus and phosphoric acid esters are added to the range between 0.1 and 1.5% 
in finished lubricants as friction modifiers. Glyceryl monooleates and Molybdenum 
compounds such as MoDTC and MoDTP also function as friction modifiers. They 
preferentially adsorb very strongly on to the metallic surface. The head of the 
friction modifier is attracted to the metal surface and the long tail with at least 10 
carbon atoms remains solubilized in the oil as shown in Figure 3 [10]. The chemical 
structure and the polarity of the molecules play a major role in the friction reduc-
tion. Ionic lubricants [11], a class of ionic liquids that are room-temperature molten 
salts consisting of cations and anions are also very good surface additives. The 
polarity of head group provides for strong surface adsorption. The physical, chemi-
cal and tribological properties of ionic liquids can be tailored to suit a wide variety 
of applications ranging from its use as polymer brushes in biological application, or 
as water soluble or oil soluble lubricant additive.
2.2.1.7 Detergents
Detergents keep surfaces free of deposits and neutralize corrosive acids formed due 
to oxidation. These molecules are chemical bases consisting of a polar substrate and 
a metal oxide or hydroxide [12]. Metallo-organic compounds of calcium and magne-
sium phenolates, phosphates, salicylate and sulfonates are recommended. Overbased 
detergents are used in marine engine lubricants to neutralize large amounts of acidic 
components produced by fuel combustion or oil oxidation. Ash (burning of organo-
metallic species) and soot particles (largely carbon with sulfur adsorbed) is formed 
by burning of the oil in internal combustion engines. Ash can then form unwanted 
residues at high temperatures or simply deposit on surfaces. The deposit precursors 
particles are insoluble in the oil and have greater affinity for detergent molecules. The 
additive molecules cling to the surface of the particle and envelop it thereby also acting 
as dispersants and prevent those particles to agglomerate and to later settle as deposits. 
A detergent additive is normally used in conjunction with a dispersant additive.
2.2.1.8 Dispersants
Dispersants are used mainly in engine oil along with detergents to keep engines 
surfaces clean and free of deposits [12]. Dispersants keep the insoluble soot particles 
and the precursors of deposits in the internal combustion engine finely dispersed 
or suspended in the lubricant even at high temperatures. These suspended particles 
are subsequently removed by oil filtration or oil change. Thus, dispersants minimize 
damage to engine surfaces and formation of high temperature deposits. Generally, 
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polymeric and ashless dispersants are used today such as polymeric alkylthiophos-
phonates, alkylsuccinimides, succinic acid esters/amides, and their borated deriva-
tives as well as organic complexes containing nitrogen compounds.
2.2.1.9 Corrosion and rust inhibitor
Corrosion and rust inhibitors are additives that reduce or eliminate rust 
(corrosion of iron and steel) and corrosion by neutralizing acids and forming 
a protective film, either adsorbed or chemically bonded on the metal surfaces. 
Preferential adsorption of polar constituent on metal surface forms the protective 
film that prevents corrosive materials such as organic acids from reaching and 
attacking the metal. These are usually compounds having a high polar attraction 
towards metal surfaces such as succinates, alkyl earth sulfonates, metal pheno-
lates, fatty acids, amines as well as zincdithiophosphates. Some of these inhibitors 
are specific to protecting certain metals. Hence, an oil may contain several types 
of corrosion inhibitors.
2.2.1.10 Extreme pressure (EP) additives
Extreme Pressure additives are required to reduce friction, control wear and 
prevent severe surface damage in heavy duty application of gears and bearings at 
high temperatures and pressures. They are also known as antiscuffing additives. They 
react chemically with metallic surfaces to form a sacrificial surface film that prevents 
the welding and subsequent seizure of asperities at the metal-to-metal contact. 
Additionally, they contribute to smoothing of the surfaces as these are formed at con-
tact asperities and the load is then distributed uniformly over a greater contact area, 
thus reducing the severity of wear and ensuring effective lubrication. Effectiveness 
of EP additives relies on their reactivity and their ability to readily form thick surface 
films at high loads and high contact temperatures that are created at the mechanical 
Figure 3. 
Adsorption of polar headgroups onto metallic surface.
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contacts. These additives usually contain sulfur and phosphorus compounds and 
chlorine or boron compounds. Ashless EP additives such as dithiocarbamates, dithio-
phospates, thiolesters, phosphorothioates, thiadiazoles, aminephosphates, phosphites 
may be preferred in some applications where chlorine may cause corrosion.
Other additives such as demulsifiers, emulsifier, biocides are added to meet 
specific requirements. Emulsifiers are used as a binder between oil and water 
molecules in oil-water-based metal-working fluids to help create a stable oil-water 
emulsion. Without the emulsifier, oil and water will separate out from each other 
due to differences in specific gravity and interfacial tension. On the other hand, 
demulsifiers are used to separate oil-water emulsions. Demulsification and removal 
of the aqueous phase from the oil-based lubricant minimizes harmful effects such as 
corrosion, foaming and cavitation from occurring. Biocides may be added to water-
based lubricants to control the bacterial growth.
2.3 Greases
Lubricating grease are a class of lubricant that do not flow like a fluid but bleed 
(release oil) when squeezed between contacting surfaces. They have a gel like con-
sistency and can be described as a solid or semifluid like material and in some cases 
can be used in vertical or overhead applications because they can have good drip 
resistance due to their Non-Newtonian rheological properties. They are particularly 
useful in applications that are sealed for life; for example bearings and remote 
gearboxes. Functionality of greases include sealing out contamination and water 
ingress, prevent corrosion, compatible with polymers and elastomers, provide 
antiwear and extreme pressure load protection while reducing friction. Greases 
have three main components: fluid, thickener and additives [13].
A typical grease consists of 75-95% fluid base stock, 2-25% thickener and 0-25% 
additives. The base stock is chosen based on the required applications. Hence, 
the fluid can be petroleum based for most automotive andindustrial application, 
synthetic based for low and high temperature application or may be wax based 
(no flow) for high load caring capacity. The thickeners are metal soaps which are 
created using the fundamental reaction of an acid and a base. Calcium based soaps 
have been the simplest and earliest used thickeners with a maximum operating 
temperature between 60 and 70°C. In the last decade, calcium sulfonate greases, 
polyurea greases, aluminum complex greases, lithium complex greases, sodium 
complex greases, and clay-based greases have received general acceptance due to 
their higher service temperatures of more than 150°C. However, each having their 
own set of pros and cons. For example, calcium-based greases do not perform well 
over a wide range of temperature, sodium based have deteriorated performance in 
the presence of water. Clay and polyurea based greases are used for high temperature 
(service temperatures of 190-220°C) and application that have limited relubrication 
access. The third component of greases are additives. Commonly used additives are 
listed below.
• Antiwear additives
• Antioxidants
• Extreme Pressure additives
• Friction Modifier
• Rust and corrosion inhibitor
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• Tackifiers (adhesive agents)
• Odorants (perfumes)
• Dyes
Tackifiers are additives that increase the adhesive property of the grease or the 
lubricant. They prevent the lubricant from flinging off the metal surface during 
rotational movement. To be acceptable to the manufacturers and the end users the 
greases must be free of offensive odor and have a desirable color. Therefore, odor-
ants and dyes are added to the grease. Although these have little effect on the grease 
performance, their appeal to senses has an impact on the product selection.
Greases have the unique ability to incorporate liquid as well solid additives. Solid 
additives such as molybdenum disulphide (MoS2), graphite, hexaboron nitride 
and polytetrafluoroethylene (PTFE) in the form of fine dispersed powder (nano 
and micro particles) have been used in lubricants and greases to provide ultralow 
friction and wear protection. Solid additives provide a physical separation between 
two contacting surfaces when fluid is unable to provide load support. The lattice 
structure of these solid lubricants plays an important role in transferring a thin low 
shear layer on the metal surfaces especially where load is high, and speed is low.
2.4 Solid lubricants
Solid lubricants on their own are vital to niche applications such as space mis-
sions, satellites release and deployment mechanisms. Vacuum and microgravity of 
space eliminates the use of liquid lubricants. The importance of a thin film of solid 
lubricant can be emphasized on the fact that the success of an entire mission can 
be compromised if the, receiver and transmitter antennas or solar arrays packed 
securely during launch fail to release smoothly with precision due to extreme fric-
tion of a deployment mechanism. Solid lubricants can be used at low temperatures 
as well as at high temperatures where the liquid lubricant may solidify or vaporize 
respectively. Even at extreme pressures where liquid lubricant will be squeezed 
out, solid lubricants are used. However, solid lubricants are not limited to extreme 
conditions [14]. A wide variety of low friction coatings are used in various 
engineering applications that require high electrical and thermal conductivities, 
low wear rates and high lubricity at all operating temperatures. Newer engineered 
coatings have increased complexity and have transitioned from single or multi 
component structures to nanostructured and functional gradient structures.
2.4.1 PTFE
The most recognized polytetrafluoroethylene (PTFE) coating is Teflon® discov-
ered in 1938 at DuPont. These are highly linear fluorocarbon molecules. They offer 
a low friction surface with moderate wear. They have low chemical reactivity and 
low surface energy. PTFE on its own performs best at low loads and wears rapidly at 
higher loads; hence they need reinforcements to increase strength and load bearing 
capacity. Such materials are called composite materials.
2.4.2 Carbon based materials
Carbon based materials such as graphite in both micro and nano forms is 
a popular solid lubricant additive whereas diamondlike carbon (DLC) makes 
excellent low friction coatings. However, they have different mechanisms of 
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friction reduction. Graphite has hexagonal crystal structure which has the intrinsic 
property of easy shear. DLC exhibits high hardness and low friction due to an 
amorphous structure that combines graphitic and diamond phases. They can be 
doped with hydrogen or nitrogen for achieving desirable properties. Recently, a 
series of patents on superlubricity of nano-diamonds and graphene films have 
been filled [15].
2.4.3 MoS2
Molybdenum disulphide are transition-metal dichalcogenides like Tungsten 
disulphide (WS2) work on the mechanism of interlamellar shear between covalently 
bonded hexagonal basal planes like that of graphite. Their performance is affected 
by moisture content [16].
Apart from the advantages mentioned earlier, solid lubricants also have a few 
disadvantages. They have less ability to carry away heat and contaminants away 
from the contact. They have poor self-healing properties, and they are not easily 
entrained into tribological contacts.
3. Application methods
After the selection of a suitable lubricant, its application method i.e. its delivery 
to the mechanical components such as gears, bearings, cams, tappets, chains, 
guideways and couplings of a machine or engine in correct quantity is considered. 
Liquid lubricants are applied to the machine elements by two methods. First type 
is called ‘all loss method’ while second type is called ‘reuse method’ [2]. In all loss 
method a small quantity of lubricant is applied periodically and the lubricant after 
use, gradually leaks away to waste. In reuse method, elaborate lubricating systems 
are designed to feed the required quantity of lubricant to various machine elements 
of the system. The lubricant after leaving the machine components is collected, 
cooled then filtered and recirculated to lubricate the machine components again. 
Most open gears, ropes, guideways, chains and rolling element bearings (except 
sealed for life rolling element bearings) are lubricated by all loss method. Nearly all 
grease lubricated elements also are lubricated by this method. Some devices used 
for all loss lubrication method include hand-held oiling device, grease gun, drop 
feed cups, wick feed cups, wick oilers, pad oilers, mechanical force feed lubricators, 
airline oilers and automatic or semiautomatic spray units. Reuse method of lubri-
cant application has the oil circulating through a network of pipes that is pressur-
ized by a pump or aided by gravity. However, closed oil sump systems employ splash 
oiling, bath oiling or ring oiling methods. Centralized lubricant application systems 
can reduce the quantity of lubricant usage as well as labour costs. Automobiles 
use an oil mist lubrication system to lubricate various machine elements of the 
engine and drive train. Considerations with regards to the lubricant characteristics 
and composition are required while designing a compatible system. Oil condition 
monitoring and routine checks are vital for the longevity of the machine elements 
lubricated by any method.
Solid lubricants on the other hand are applied as powder dispersed in a liquid 
lubricant or in a solid phase matrix. They can be also applied as a thin film coating. 
The coatings are made by various methods such as dip coating, thermal spraying, 
and cold. More robust ways of coating includes chemical vapor deposition or physi-
cal vapor deposition methods. Electrochemical processes are used for producing 
coatings of polymer-based solid lubricants and theircomposites.
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4. Lubricant analysis
Lubricant analysis primarily refers to the characterization and evaluation of the 
lubricant for various physical, chemical and performance properties in all stages 
of its life cycle. For solid lubricants and coatings, the analysis includes determining 
the composition and structure by using a range of spectroscopic and microscopic 
observation methods as well as measuring the coating consistency and thickness 
applying non-destructive evaluation techniques. Standardized tribological lab tests 
are used to evaluate their performance. Once a candidate material is identified, 
larger-scale bench testing, such as engine tests, are conducted.
For liquid lubricants, characterization is done to a much larger extent due to the 
large variety of applications and the implication of chemistry of the formulated oils 
on the machine elements performance and the overall performance of the entire 
equipment/machine or engine. The lubricant analysis can be classified as physical 
and chemical characteristics evaluation, Performance test evaluation and Engine 
Test evaluation as summarized in Table 2 [2]. Additionally, end users also develop 
their in-house in-service lubricant analysis to monitor and maintain the condition 
of the lubricant. The objective of such analysis is to ensure optimum performance, 
achieve expected life of the equipment as well as the lubricant flowing through it.
5. Future trends
Current automotive lubricants are optimized for internal combustion engines and 
drive trains. Electric vehicles (EVs) which use electric motors possess new challenges 
of lubrication such as high-power density of the small gear box which require efficient 
cooling. Hydro lubricants and synthetic gear oils are excellent candidates for such 
application but may pose sealing issues which requires innovative solutions. Lubricants 
are also required in the rolling element bearings of EVs that must stop electro-erosion 
Physical and chemical 
characteristics
Performance test 
evaluation
Engine test evaluation
Color Oxidation Tests Oxidation stability and bearing corrosion 
protection
Density and API gravity Thermal Stability Single cylinder high Temperature tests
Carbon Residue Foaming Tests Multi cylinder high temperature tests
Flash point Corrosion and Rust 
Protection Test
Multi cylinder low temperature tests
Neutralization Number EP and Antiwear Test Rust and corrosion protection tests
Total Acid Number Emulsion and Demulsibility 
Test
Oil Consumption rates and volatility
Total Base Number Emission and protection of emission 
control systems
Pour Point Fuel Economy
Sulphated Ash
Viscosity
Volatility
Table 2. 
Evaluation techniques and testing.
Tribology
14
caused by high frequency high energy discharges by being conductive. Ionic liquids are 
having shown good performance in preventing built up of potential [17].
The future for lubricants formulation, manufacturing and end-use is oriented 
towards efficiency in terms of cost and time, customized and optimized for each 
individual tribo-system and run reliably for even longer drain interval time. New 
industrial lubricants must meet stringent regulations and guarantee ecological 
sustainability, and climate change actions. Hence, new lubricants will contribute to 
‘Green Tribology’.
Tribology of lubricants plays a significant role in technology and economics of 
industrial development. As the fourth industrial revolution ‘Industry 4.0’ (com-
bines automation with the internet of things) is in progress, several concepts can 
be extended to lubricant and lubricant additive development and evaluation. For 
example new electronic smart sensors can be used for lubricant analysis and condi-
tion monitoring. The information of various performance parameters can be stored 
as data and transferred to the stakeholders and decision-making points using the 
information and communication technologies involved in Industry 4.0. such as 
internet and wireless connections to and via several electronic devices. Continuous 
monitoring can also aid in corrective measures. In-service lubricant performance 
testing and evaluation generates big quantities of data from various equipment 
and sensors. Therefore, ‘Big Data’ concepts can be applied in several ways. One 
such example can be combining lubricant data with machine data, another can 
be correlation study of amount of soot produced, change in oil viscosity, friction, 
wear of an engine. The benefits of Industry 4.0 include shorter lubricant develop-
ment time, reduced number of trials, lubricant performance prediction through 
chemical and physical modeling and simulations. This will transform product 
development process from being empirically driven to using data and simulation 
driven approach.
6. Conclusion
Lubricants are vital for the tribological life of the machine elements. As modern 
machine elements are required to perform in heavy-duty applications in a wide 
range of environments, newer, better and environmentally sustainable lubricants 
are required to be designed. Lubricant additives are therefore considered as lubrica-
tion engineering design components. Lubricant additives are designed and opti-
mized to meet the performance requirements of the equipment or engine. Various 
types of lubricant additives and their functional properties were discussed. They 
are designed to provide oxidation resistance, high temperature viscosity, energy 
and fuel efficiency among others. Lubricant or grease therefore are a complex 
mixture of several components blended carefully together to meet the performance 
requirements. The different components can have synergistic or antagonistic 
effects due to chemical interactions or competition at the metal surface or among 
themselves. Therefore, formulation of lubricants requires considerable expertise 
and expensive performance testing. Green Tribology and Industry 4.0 era will steer 
the lubricant development, use and disposal.
Acknowledgements
This chapter could not be published without the exceptional support of Prof. 
Paul B. Davies (University of Cambridge, UK) and substantial industry inputs and 
scrutiny by Mr. Tony D. Smith (Castrol, UK).
15
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DOI: http://dx.doi.org/10.5772/intechopen.93830
Author details
Debashis Puhan
Department of Chemistry, University of Cambridge, Cambridge, United Kingdom
*Address all correspondence to: dp617@cam.ac.uk
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms 
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, 
provided the original work is properly cited. 
16
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